Solarium Ore is used to craft Hydrolyth bars and Solarium Bars, crafted from 2 Solarium Ore.
How to Get Solarium Ore
In order to get the Solarium Ore you will need to defeat Golem to get an item called the Totem of the Golem. It will teleport you to the Hidden Temple, made of Blue Lihzard Brick. In the room there is an altar and two chests containing various items such as elemental shards, Golem items and other misc items. At the altar, you will need Earth Stones dropped by Golem and a Lihzard Power Cell to craft a Solarium Rod. Then you can right click the Solarium Altar with the Solarium Rod, and that will subsequently generate Solarium ore in your world at cavern level and below.